The invention relates to alarm systems and, more particularly, to an alarm system for mobile construction equipment to detect potential hazards due to overhead power transmission lines.
Overhead electric power lines present a serious electrocution hazard to personnel in a variety of industries. Overhead lines, typically uninsulated conductors supported on towers or poles, are the most common means of electric power transmission and distribution, and are exposed to contact by mobile equipment such as cranes and trucks. Equipment contacting energized overhead lines becomes elevated to a high voltage, and simultaneous contact by personnel to the xe2x80x9chotxe2x80x9d frame and ground can cause serious electrical shock and burns. Industries where risk of these accidents is greatest include construction, mining, agriculture, and communications/public utilities. An estimated 2300 accidental overhead line contacts occur each year in the U.S.
Construction activities present the most obvious potential for line contact accidents, and a recent study estimated that in 1993 alone, at least 26 electrocutions in this industry were a result of heavy equipment or hoisted loads contacting overhead lines. Mobile cranes (including boom trucks) were involved in most of these incidents (57%), with drill rigs (8%), dump bed trucks (7%), and manlifts (7%) also common. Detailed and more comprehensive statistics are available for the mining industry, which represents a smaller work force than the construction industry, but has a similar electrocution rate, and like construction uses heavy equipment extensively. From 1980 to 1997, at least 94 mobile equipment overhead line contact accidents were reported in the U.S. mining industry, with 114 injuries, 33% of them fatal. Most involved cranes (47%), dump bed trucks (24%), and drills (14%).
Proximity warning devices are known which purport to warn against impending contact with power lines. A significant shortcoming is that proximity warning devices are based only upon electrostatic field sensing which can fail to alarm reliably under certain configurations of multiple power line circuits. Other prevention techniques include de-energizing lines, maintaining appropriate distances from energized lines, use of an observer to warn the operator of impending contact, and barriers to prevent physical contact with an energized line. Oftentimes, however, these methods are not practical due to over-reliance on field personnel or because of the expense associated therewith. Another approach is the use of insulating links in the load line which attempts to prevent injury once contact has been made. However, surface contamination and moisture can reduce the insulation resistance of these insulating links and workers who contact parts of the equipment other than the load will not be protected.
Accordingly, there is a need for an alarm for mobile equipment that warns of the potential for electrocution without relying on sensing of power line electrostatic fields. A low cost and reliable alarm for warning personnel as to electrocution hazards would be desirable.
In accordance with the present invention, an alarm system is provided that utilizes a sensor for detecting power at the equipment which signals an alarm to warn of a potential shock or electrocution hazards when present. In this regard, the sensor does not rely upon detection of the power line electromagnetic field (emf), per se. Instead, the present invention detects the current flow on the equipment and signals its presence by causing or activating an alarm unit to warn of the potential hazard at the equipment.
In one form of the invention, an alarm system is provided for use with equipment of conductive material through which current can flow when electrical power is transmitted thereto. The alarm system includes a sensor for generating a signal in response to the presence of outside electrical power at the equipment due to current flow on the equipment. An alarm unit is provided for receiving the signal and providing one of a visual and audio indication that the electrical power is present. Because the present sensor signals the presence of current flow on the actual equipment and not the power line electrostatic or electromagnetic field, there is improved reliability as the functionality of the present invention does not depend on the configuration of the power lines.
The sensor can include a conductor with the signal from the sensor being generated in response to the presence of a predetermined amount of current flow through the conductor.
In one form, the conductor is attached to the equipment for establishing a path of least resistance to direct the current therethrough. The conductor can be a sensor cable that is easily welded or otherwise attached to the equipment to provide a low cost sensor of the present invention.
In another form, the sensor is an inductive sensor and the conductor is attached to the equipment with the inductive sensor detecting current flow through the conductor corresponding to the presence of a threshold level of potential on the conductor.
The inductive sensor can electrically isolate high levels of power at the equipment therefrom for ensuring the signal is generated despite the presence of the high power levels. In this manner, the present inductive sensor is well-suited for sensing power transmitted to the equipment from high voltage power transmission lines (e.g., 100,000 volts or more) minimizing the risk of sensor failure.
In one form, the conductor is attached to surfaces of the equipment that are adapted to move relative to each other.
The equipment may be provided in combination with the alarm system with the equipment being a mobile vehicle having moving portions that can engage overhead power lines such as when the vehicle is driven and/or as the portions are moved relative to each other for transmitting power to the portion of the vehicle in engagement with the power line and through other portions of the vehicle.
The sensor can include a conductor that is one of the portions of the vehicle with the signal being generated due to a predetermined current through one portion of the vehicle. In this manner, the present invention can be easily retrofit to the vehicle with a minimum of expense.